A lithium tantalate crystal is a ferroelectric substance, with a melting point of approximately 1650° and a Curie point of approximately 600°, which has piezoelectric properties. Lithium tantalate substrates manufactured from lithium tantalate crystals are used primarily as a material for surface-acoustic wave (SAW) filters used to accomplish signal noise rejection in mobile phones. Factors such as the use of higher frequencies with mobile phones, and the proliferation of Bluetooth (2.45 GHz) as a wireless LAN for a variety of electronic equipment, mean that from now on a rapid increase in demand is anticipated for SAW filters in and around the 2 GHz frequency domain.
In the construction of a SAW filter, a pair of comb electrodes made of a metallic thin film produced from an AlCu alloy or the like, are formed on a substrate made of a piezoelectric material, for example a LT substrate. These comb electrodes have the important function of controlling the polarity of the device. The comb electrodes are formed by depositing a metallic thin film on the piezoelectric material by a sputtering method, and leaving a pair of comb shaped patterns while etching away the unwanted portions using photolithographic techniques.
To be compatible with even higher frequencies, it is necessary for the comb shaped pattern to be fine, as well as thin. Compared to devices operating in and around the 800 MHz frequency domain, which is currently the mainstream, devices operating in and around the 2 GHz frequency domain require a distance between electrodes approximately one third as wide, that is between 0.3 μm and 0.4 μm, and a film thickness less than one fifth as thick, that is below 200 nm or thereabouts.
Industrially, LT crystals are grown using the Czochralski method, normally using a high-melting iridium crucible inside an electric furnace, under a nitrogen-oxygen mixed gas atmosphere with an oxygen concentration between a few percent and 10% or thereabouts, and the crystals are removed from the electric furnace after being cooled at a predetermined cooling rate inside the electric furnace (Albert A. Ballman: Journal of American Ceramic Society, Vol. 48 (1965)).
The process of manufacturing a wafer from a LT crystal involves crystal growing (ingot), poling, cylindrical grinding, slicing, lapping, polishing and wafer completion, in that order.
Specifically, a grown LT crystal is either colorless and transparent, or exhibits a highly transparent pale yellow color. After the LT crystal is grown, heat treatment of the crystal is performed under an even temperature near the melting point to remove residual strain in the crystal caused by thermal stress. In addition, poling is performed to obtain a single polarization. In other words a series of processes is performed involving; heating the LT crystal from room temperature to a predetermined temperature above the Curie point (approximately 600° C.), applying a voltage to the LT crystal, and while applying this voltage, lowering the temperature of the LT crystal to a predetermined temperature below the Curie point, and subsequently stopping voltage application and lowering the temperature of the LT crystal to room temperature. After poling, the LT crystal ingot, which has undergone cylindrical grinding to prepare its external form, is sliced to form wafers, and these wafers undergo machining including lapping and polishing and the like, to obtain LT substrates. The thus obtained LT substrates are nearly colorless and transparent, and have extremely low electrical conductivity, at approximately 10−13 S/m (volume resistivity 1015 Ωcm).
A LT crystal, which is a ferroelectric substance, also has pyroelectric properties. Accordingly, when LT substrates are obtained by conventional methods, the temperature variations sustained during the surface acoustic wave device manufacturing process can cause an electric charge to accumulate on the surface of the LT substrate due to the pyroelectric properties of the LT crystal, and this charge can generate sparks. These sparks can destroy the comb pattern formed on the surface of the LT substrate, and cause cracking or the like of the LT substrate, which reduces yield during the surface acoustic wave device manufacturing process. Moreover, because the electrical conductivity of the LT substrate is extremely low as mentioned above, the charge-build-up state is maintained, and a state in which sparking can occur easily continues for extended periods of time.
Furthermore, because the LT substrate has high light transmittance, a problem occurs in that the light which passes into the LT substrate during the photolithographic process, which is one part of the surface acoustic wave device manufacturing process, is reflected from the rear surface of the LT substrate back onto the front surface, causing deterioration of the resolution of the comb pattern formed on the substrate.
In order to solve these problems, in Japanese Patent Publication No. Tokukai Hei 11-92147 and Japanese Patent Publication No. Tokukai Hei 11-236298, a solution is proposed whereby lithium niobate (LN) crystals are exposed to a reducing atmosphere of argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide or a mixture of gases selected from this group, at a temperature range of between 500 and 1140° C., thereby blackening the LN crystal wafers and thus controlling the high light transmittance of the LN substrate, while increasing the electrical conductivity, so that light reflected back from the rear surface of the LN substrate is suppressed, and at the same time the pyroelectric properties of the LN substrate are reduced.
Although these publications refer to LT crystals as well as LN crystals, there are no substantial disclosures relating to LT crystals.
Furthermore, according to experiments carried out by the inventors of the present invention, it was discovered that the methods disclosed in these publications were effective with LN crystals that had a low melting point of approximately 1250° C., but had no effect with LT crystals that had a high melting point of approximately 1650° C.
In Japanese Patent Publication No. Tokukai 2004-35396 (WO2004/002891A1), it is disclosed that LT crystals are susceptible to a charge-build-up state caused by an electric charge produced by heat or mechanical stress, and for devices which use LT crystals, from a stability viewpoint it is necessary to dissipate this charge, and that with LN crystals, the heat treatment under a reducing atmosphere causes electrical conductivity to increase, allowing charge-build-up to be prevented, and also that the same effects cannot be obtained for LT crystals as were obtained for LN crystals using the same methods.